Industrial rated molded case circuit breakers find application over ampere ratings ranging from 50 to 1,200 amperes for any common electronic trip unit design. To set the ampere rating, a burden resistor is electrically connected within the trip unit circuit for generating a voltage representation of the circuit current. To facilitate manufacture and inventory efficiency, a removable rating plug is often employed to allow the burden resistor to be varied without requiring customized tailoring of each trip unit circuit for each circuit breaker ampere rating.
U.S. Pat. Nos. 3,440,491; 4,007,401; 4,181,922 and 4,368,500 all teach removable rating plugs for setting the circuit breaker ampere rating.
Another means of adapting circuit breakers for various ampere ratings comprises changing the current sensing transformer taps to provide the requisite voltage representation of circuit current. U.S. Pat. Nos. 4,181,389 and 4,281,359 disclose means for such adjustment of the current sensing transformer taps.
In order to conform with industry standards, the trip unit circuit must either automatically trip or else default to not more than lowest ampere rating when the rating plug is removed from the trip unit circuit. The simplest known method for providing such a default option is to electrically connect the burden rating resistor carried by the rating plug in parallel with a fixed burden resistor electrically connected within the circuit. Upon removal of the rating plug, the fixed burden resistor then defines the lowest ampere rating of the trip unit circuit.
With the advent of high speed electronic trip unit modules by large scale integration techniques, such as described within U.S. Pat. No. 4,589,052 entitled "Digital I.sup.2 T Pickup, Time Band And Timing Control Circuit For Static Trip Circuit Breakers", in the name of John Dougherty, it is economically efficient to assemble the rating plug components to the rating plug and to assemble the rating plug to the trip unit circuit board in an automated assembly process.
One purpose of the instant invention is to provide a rating plug wherein the burden resistors and current sensing transformer taps can be economically assembled to the rating plug. A further purpose of the invention is to provide for grounding the current sensing transformer outputs before removing the rating plug from the trip unit circuit. Additionally, the invention proposes to reduce the circuit breaker rating to a minimum default value upon removal of the rating plug from the trip unit circuit. A further purpose of the invention is to provide for trip function selection similar to that described in U.S. patent application Ser. No. 760,224, filed July 29, 1985, entitled "Electronic Circuit Breaker Trip Function Adjusting Circuit" in the names of Graham Scott et al., now abandoned whereby specific overcurrent pickup values can be individually selected.